1. Field of the Invention
The present invention relates generally to a thin film stacked crystal filter and, more particularly, to a 2-port multipole filter comprised of a series of cascaded thin film stacked crystal filters.
2. Discussion of the Related Art
Semiconductor bulk acoustic resonators (SBARs), well known to those skilled in the art, are thin film acoustic resonators fabricated on a semiconductor substrate. SBARs typically operate similar to a conventional bulk crystal resonator, but are much thinner and resonate at a much higher frequency. An SBAR will generally include a thin layer, or film, of a piezoelectric material, such as zinc oxide or aluminum nitride, with conducting thin film electrode layers fabricated on opposite surfaces of the piezoelectric layer. The thin film piezoelectric layer is generally formed by a sputtering process on a suitable semiconductor substrate. Such a fabrication technique enables an SBAR to be integrated into semiconductor circuits including oscillators, filter-amplifiers, receivers, etc.
It is known to provide stacked SBARs where electrodes are positioned above, between and below multiple piezoelectric layers to form a 2-port stacked crystal filter (SCF). A SCF incorporating multiple SBARs of this type operates as a complete filter for producing a narrow band frequency output. These types of filters, however, do not have output passband shapes controllable by design and therefore are limited in their application. For a review of devices incorporating SBARs, see Cushman, D., et al., "SBAR Filter Monolithically Integrated with HBT Amplifier," Proceedings 1990, IEEE Ultrasonic Symposium and Kline, G. R. et al., "Thin Film Microwave Acoustic Filters on GaAs," IEEE GaAs IC Symposium, 1988.
Multipole filters incorporating 1-port SBARs are also known in the art. One example is disclosed in Driscoll, M. M., et al., "Recent Advances in Monolithic Film Resonator Technology," Ultrasonic Symposium, 1986, pp 365-369. Driscoll discloses a multipole filter comprised of a series configuration of 1-port SBARs including inductors electrically connected to ground between the SBARs. The Driscoll approach, however, suffers from direct feedthrough through the capacitance of the individual 1-port SBARs. This feedthrough is cancelled at the center frequency by adding a shunt inductor L.sub.o in parallel with each SBAR. However, this inductor doubles the number of tuning components of the system and is not effective at out-of-band frequencies as can be seen by FIG. 5 in that document.
Also known in the art are 2-port multipole filters incorporating surface acoustic wave (SAW) resonators as disclosed in Schreve, W. R., "Surface-Wave-to-Port Resonator Equivalent Circuit." Because this approach uses the surface acoustic wave technique, and not the SBAR technique, it cannot realize the advantages associated with SBARs. The advantages of the SBAR which are not achieved with the SAW approach include: (1) integration with active circuits on semiconductor substrates; (2) small size; and (3) frequencies greater than 2 GHz.
What is needed then is an effective SBAR multipole filter which provides narrow band filtering and which has passband shape and rejection characteristics controlled by design. It is therefore an object of the present invention to provide such a filter.